| 摘要: | 本文於2014年春季在鹿林山背景觀測站(以下簡稱為鹿林山，2,862 m)以即時氣膠水溶性監測儀(Particle-Into-Liquid-Sampler coupled to an Ion Chromatograph, PILS-IC)監測PM2.5氣膠水溶性無機離子(WSIIs)，並收集測站監測的PM2.5、PM10氣膠質量濃度、PM10、PM1氣膠散光及吸光係數、氣膠粒徑分布、氣膠總數目、氣體污染物動態變化。
鹿林山正好位於盛行西風下風處，適合觀測春季中南半島生質燃燒煙團及亞洲大陸污染傳輸。本文監測不同污染事件短時間氣膠水溶性無機離子動態變化，同時評估大氣SO2與NH3溶解在雲霧中形成的WSIIs濃度，以瞭解雲霧中未被活化和霧珠中氣膠水溶性無機離子動態變化。
當鹿林山逆推軌跡來源為自由大氣，各項污染物濃度都很低。NH4+、NO3-、SO42和K+分別為0.1 ± 0.4、0.2 ± 0.4、0.7 ± 0.7和0.0 ± 0.0 μg m-3，應可代表東亞大氣在生質燃燒好發期間乾淨背景大氣的WSIIs濃度。然而當受到生質燃燒影響時，CO、PM2.5質量濃度、PM1綠光散光、吸光係數都有增高現象。NH4+、NO3-、SO42和K+分別達到1.9 ± 1.5、1.0 ± 1.0、3.6 ± 2.9和0.2 ± 0.2 μg m-3高濃度。從無機離子相關性分析，顯示這些氣膠化合物型態以硫酸銨、硝酸銨為主。
當發生雲霧事件時，CO、NOx有隨時間上升的趨勢，NH4+、NO3-、SO42-無機離子濃度明顯上升，觀測結果雲霧多為山下氣流抬升形成。NH4+與SO42-有中高程度相關性(R2>0.64)，推論這兩種無機離子結合型態可能為硫酸銨或硫酸氫銨。從ExNO3-與ExNH4+計算判斷NO3-生成機制，發現在不同時段分別有HNO3(g) 凝結在氣膠、N2O5水解以及硝酸銨氣膠(Mwaniki et al., 2014)。另外發現微粒粒徑分布的眾數粒徑在雲霧中有由小逐漸變大的傾向，顯示雲霧中有氣膠膠凝成長的現象。
本文使用PILS-IC量測的SO42-和NH4+以及假設大氣SO2和NH3與霧珠中溶解的SO42-和NH4+達成平衡，計算DIGMI (Dissolved Gas over Measured Ions)值。SO2在一般雲霧中DIGMI值大多接近1.0，顯示有不錯的氣體轉化現象，然而受到生質燃燒煙團影響的雲霧中會有大量氣膠使DIGMI值低於1.0或SO2溶於霧珠中使DIGMI值遠大於1.0。NH3方面，不管是否受到生質燃燒煙團影響，雲霧事件中平均DIGMI值都大於1.0，代表NH3在霧珠中溶解度高。;This work monitored short-term variations of water-soluble inorganic ions (WSIIs) of atmospheric PM2.5 using Particle-Into-Liquid-Sampler coupled with an Ion Chromatograph (PILS-IC) at the Lulin Atmospheric Background Station (LABS, 2,862 m a.s.l.) in spring 2014. In addition, PM2.5 and PM10 mass concentrations, PM10 and PM1.0 scattering and absorption coefficients, aerosol size spectra, aerosol total number concentration, and dynamic variations of gaseous pollutants were also monitored at LABS.
Under prevailing westerly, LABS is a proper site for observing transported biomass burning (BB) smoke and Asian continent pollution due to its downstream location of the air masses transported from Southeast Asia. In this study, short-term dynamic variations of WSIIs were observed during different pollution events. Meanwhile, formations of WSIIs from dissolved atmospheric SO2 and NH3 in fog were evaluated to investigate dynamic variations of WSIIs in inactivated aerosol and fog droplets during fog.
All pollutant levels were very low when backward trajectories were originated from free atmosphere. The concentrations of NH4+, NO3-, SO42, and K+ were 0.1 ± 0.4, 0.2 ± 0.4, 0.7 ± 0.7, and 0.0 ± 0.0 μg m-3, respectively, which may represent clean atmospheric background of East Asia during extensive BB period. In contrast, CO, PM2.5 mass level, PM1 green scattering and absorption coefficients were all enhanced when affected by the transported BB smoke. The corresponding concentrations of NH4+, NO3-, SO42, and K+ were 1.9 ± 1.5, 1.0 ± 1.0, 3.6 ± 2.9, and 0.2 ± 0.2 μg m-3, respectively. From linear correlation analyses of these chemical components, ammonium sulfate and ammonium nitrate are their possible compound forms.
The levels of CO, NOx, NH4+, SO42-, and NO3- were observed to increase with time during fog event periods at LABS. It suggests that ground-level pollutants were transported by the uplift flow to form fog at LABS. Moderately high linear correlation between SO42- and NH4+ (R2>0.64) indicated that the compound form of these two ions might be ammonium sulfate or ammonium bisulfate. From the calculation of ExNO3- and ExNH4+, three nitrate formation pathways (Mwaniki, et al., 2014) can be inferred to be condensed HNO3(g) onto aerosol surface, N2O5 hydrolysis, or the formation of ammonium nitrate particles in different times. Moreover, the modal diameter of aerosol size spectra was gradually increased in the fog events which showed a coagulation growth of aerosol in the fog.
A value of DIGMI (Dissolved Gas over Measured Ions) was calculated from measured SO42- and NH4+ concentrations by PILS-IC and the dissolved SO42- and NH4+ concentrations in fog droplets in equilibrium with atmospheric SO2 and NH3. The DIGMI values of SO2 were mostly close to 1.0 during normal fog events indicating good gas dissolution. However, the DIGIMI values varied greatly when affected by the transported BB smoke. For NH3, the mean DIGIMI values were both greater than 1.0 with/without an effect of the transported BB smoke implying a good dissolution of NH3 in fog droplets. |
